This invention relates to printing machines generally, and more particularly to rotary sheet printing machines.
Still more particularly, the invention relates to rotary sheet printing machines which are equipped for selectively printing on the obverse side of a sheet or on the obverse and reverse side thereof.
Machines of the general type in question are known. For example German Democratic Republic Patent No. 54,704 discloses a rotary printing machine having a pair of printing stations between which there is either located a transfer roller or a reversing roller. If the machine is to print only the obverse side of a sheet, it comes equipped with a transfer roller which simply passes the sheet from one to the other of the printing stations, both of which print on its obverse side. If the machine is to print on the obverse and reverse sides--i.e. perfection printing--it comes additionally equipped with a reversing roller which receives the sheet from one printing station which has printed on the obverse side, reverses the sheet and presents it to the other printing station in readiness for printing by the same on the reverse side.
Analogous printing machines are disclosed in U.S. Pat. Nos. 3,537,391 to Mowry and 2,757,610 to H. W. Gegenheimer et al, except that Gegenheimer et al discloses a machine having three rollers of which one is a reversing roller and which are all three located between the printing cylinders of two successive printing stations, whereas GDR Patent No. 54,704 and Mowry each disclose a single roller which operates as a reversing roller and is located between the printing cylinders of two successive printing stations.
These machines are basically satisfactory. However, further improvements are needed because of economic and operational considerations.
When prior-art machines are used for high-speed obverse-reverse printing, there is insufficient time to dry the ink applied to the obverse sheet side before this side, upon reversal of the sheet, comes into contact with the rollers of the next-following printing station. The inherent--and inevitable--result is a deterioration of the print quality on the obverse side. This is evidently undesirable and, in the case of perfection printing, completely unacceptable. Yet, economic and other considerations dictate ever higher printing speeds, so that this problem is becoming more and more prevalent.
In addition, economic considerations, also play a part in the manufacture of such machines. Depending upon whether the machines are to be used only for obverse printing, or for obverse-reverse printing, they are equipped with either only a sheet transfer roller or with an addional sheet reversing roller, as already explained. These rollers are an integral part of the respective machine. The different machines must be built on different assembly lines because the type of intermediate roller used--i.e. whether a sheet-transfer or a sheet-reversing roller--influences the construction of the printing stations, meaning that printing stations which cooperate with a reversing roller will be structurally different from those which cooperate with a transfer roller. This increases the construction cost and makes it difficult to change manufacturing dispositions in accordance with changes in customer instructions which are often received on short notice.
Also, if the machine employs a sheet reversing roller which is of the type that engages the trailing edge of the sheet received from the upstream printing station, it requires special devices to assure that there will be an adequate sheet length between the successive printing stations. This is necessary because it must be assumed that at the moment at which the trailing sheet edge is gripped by the gripper mechanism, the leading sheet edge has not yet entered the nip between the sheet reversing roller and the output roller of the upstream (i.e. preceding) printing station, since damage to the sheet is otherwise likely to result.